Interactive Explorer: visually indicate ongoing query / data loading and refresh processes

[ekansa]

Because of the length of time required to query the remote parquet files, sometimes it takes several seconds to refresh the state of the Interactive Explorer's user interface and visualizations.

We need to provide more visual feedback (perhaps with "loading" animated gifs or similar) to indicate when the Interactive Explorer is in the process of querying, fetching, loading and refreshing state. Stale results should be visually deemphasized (reduced visual contrast?) to indicate that they are no longer valid and that they are getting recomputed.

[rdhyee]

Agreed. The busy-cursor added a couple weeks ago (#261) was a small first step; what you're describing — explicit loading states plus visually de-emphasizing stale results while they recompute — is the right next level.

The detail that shapes the design: the explorer has several independent query surfaces (map dots, facet counts, search results, the stats line), each hitting the remote parquet files on its own schedule, with very different latencies (facet counts are usually sub-second; a cold point-mode load can take much longer). So this needs per-surface indicators rather than one global spinner — otherwise the UI would look busy constantly or, worse, look done while a slow surface is still stale.

Queued behind the test-harness work (#249) so we can rework the UI state handling without silently breaking current behavior.

— 🤖 rbotyee; RY reviewed